Rotatable coaxial joint



Dec. 22, 1959 R. L. CATTOI ETAL ROTATABLE COAXIAL JOINT Filed May 22. 1957 INVENTORS HARRY W. LEHMAN ROBERT L. CATTOI BYW ATTORNEYS ROTATABLE coAxmL JOINT Robert L. Cattoi, Cedar Rapids, and Harry Lehman, Marion, Iowa, assignors to Collins Radio Company, Cedar Rapids,'Iowa,' a corporation of Iowa Application May 22,1957, Serial No. 660,828

I 1: Claim. cl. 333-47 This invention relates in general to the electrical connection between tworelatively rotatable bodies and more particularly to rotatable joints for coaxial cables.

Prior forms of rotatable coaxial joints involved sliding finger or brush systems or continuous cable connections forbidding continuous rotation beyond some predetermined limit; In high performance applications, a joint capable of rotation of one portion relative to the other with very low staticfriction- (stiction), low leakage, and good voltage standing wave ratio characteristics, as well as a constant impedance as a function of rotation has presented a considerable problem, especially with a system capable ofcontinuous rotation. Prior systems may haxe been satisfactory for low rotational speeds, low duty cycles, or applications where stiction was no object, but in general, these systems have been unsatisfactory due to variation of their electrical characteristics throughout by a hyperboloid spring-are used to make part of the electrical connection.

It is a further feature of the invention that the assembly is inherently capable of smooth transitions in size yielding desirable electrostatiefield due mechanical complication. 7

Further objects, features, and-advantages of the invention will become apparent from the following description and claim when read in conjunction with the accompanying drawing in which the figure shows a half-sectional isometric view of the invention.

In the figure the coaxial joint'ismounted on an upper mounting flange 10 and a lower mounting flange 11. These two mounting flanges are fastened to opposing portions of the equipment between which rotation must occur. The center of rotation of one portion of the equipment relative to the other is coincident with the center of mounting flanges 10 and 11.

The upper mounting flange 10 has a slot 12 formed therein for the accommodation of a coaxial cable 13. For applications requiring an axial connection, the slot 12 may be omitted using, however, the hole 14 with an extension of the hole to both conductors of the joint. In the present form, the center pin or conductor 15 is fastened to the mounting means 10. The mounting means 10 is formed of an insulator material having the desired characteristics for the frequency used.

Center conductor 15 is maintained rigidly in its mounting by knurling, staking, or gluing in some fashion. The center conductor of the coaxial cable 13 is soldered to the upper end of center pin 15.

patterns without un- 2,918,638 Patented Dec. 22, 1959 An outer conductor 16 in the form of an annulus of metal is also mounted on the insulating flange 10. The outer conductor 16 is concentric to the inner or center pin 15. The inner radius of the coaxial conductor 16 is proportioned relative to the radius of the center pin taking into account the dielectric constant of the insulating body 10 to set the desired characteristic impedance of this portion of the joint equal to that of the coaxial cable 13.

The outer conductor 16 has a bore 17 traversed by a set screw 18 for connection with the outer conductor or braid of the coaxial line 13. Thus, connection of this portion of the joint to the coaxial line leading away from it is made by soldering theinner conductor and clamping the outer conductor by set screw 18.

The opposite portion of the coaxial joint is mounted on the lower flange 11. Flange 11 alsois an insulating body. A slot 22 is formed therein to accommodate a second coaxial cable 23 which leads away from the lower portion of the coaxial joint. As before, for applications requiring axial positioning of the coaxial cable 23, the hole 24 only may be used.

Extending into hole 24 is a center conductor 25. The inner conductorof cable 23 is soldered to this center conductor. The radius of the center pin'25 is approximately the same as that of the upper center pin 15. Between the upper and lower'ceuter conductors 15 and 25, respectively, IS a ball 30 which makes electrical contact between the two conductors.

The lower center conductor 25 -is carried by an insulating cylinder 31. The center conductor 25 is fastened to the insulator 31 by knurling, staking or-gluing or some other; means; Where ceramic insulators are used; the

center pin canbe inserted. prior to firingof the ceramic.

Insulator-3'1 has substantially the same outside diameter as. the outer diameter of the dielectric of'frame 10 inside the annulus-or upper outer conductor 16. The inside bore of the insulator 31, where it overlaps the upper center conductor 15,. is provided with'acle'arance to permit free motion upon rotation of the two sections of the joint.

Between a shoulder 32 and a flange 33 of the lower insulating block 11 is mounted a coil spring 34. This coil springprovides sufiicient contact pressure for proper electrical connection betweenthe lower center pin 25, the contact ball 30, and the upper center 'pin 15. The strength of this coil spring is regulated to provide sufficient pressure for reliable electrical contact but not so high as to increase the stiction of the joint appreciably. Spring member 34 is initially compressed so that expan-' sion thereof forces insulating member 31 and conductor 25 upward, bringing the conductor into contact with ball 30.

Just outside of the insulating block 31 and the spring 34 is a lower outerconductor 36. The internal bore of this lower outer conductor is approximately that of the inside bore of the upper outer conductor 16. The lower outer conductor 36 is mounted in insulating block 11.

The insulating block 31 and the lower outer conductor 36 extend to but do not quite rub against the adjacent surfaces of the upper outer conductor 16 and the upper insulator 10. This interface space is made as close as desirable taking into consideration manufacturing tolerances.

Also mounted on the insulating block 11 is a washer 37. This washer fits snugly against the outer conductor 36 and is arranged to make electrical contact therewith. Mounted on washer 37 is a spring 38. This spring is a hyperboloid of one sheet. The spring is formed of a single unitary thickness of metal. One portion of it, the upper portion 39, is smooth and continuous, in the form of a raceway. The lower portion 40 resting on the spring backing washer 37 is slitted to provide further resilience.

Opposite the raceway portion 39 of hyperboloid spring 38, the adjacent face 41 of upper outer conductor 16 is also formed as a raceway. Ordinarily the raceway 41 is formed, as shown, of conical shape. For applications requiring some angular displacement transverse to the axis of rotation, in the manner of a self-aligning type of bearing, the raceway surface 41 of upper outer conductor -16 should be spherical with the center of the sphere at the center of ball 30. Of course, the clearances between the lower outer conductor 36, the insulator block 31, and the upper center pin should be sufiicient to permit the desired angular deviation.

A plurality of balls 42 are deployed around the circumference between the two raceways 39- and 41. In order to maintain these balls in a spaced relationship, the upper portion of the lower outer conductor 36 adjacent the interface area is extended into radial fins 43 to form a separator for said plurality of balls. Thus, a much smaller number of balls may be used to complete the electrical contact between the upper outer conductor 16 and the hyperboloid spring 38. Further, the interface space is minimized, adding a capacitive shunt to the gap, reducing its effect electrically.

In use, the rotatable joint makes an electrical connection between the outer conductor of the coaxial cable 13 to the upper outer conductor 16, through the plurality of balls 42 to the hyperboloid spring 38, from the spring to the spring backing washer 37, then to the lower outer conductor 36 and finally to the braid or outer conductor of the coaxial 23. This circuit is the reliable one, especially so in view of the spring pressure of the hyperboloid spring 38. Accidental contact between the balls 42 and the separator portions 43 of the inner conductor 36 are fortuitous but not relied upon. The proximity of the upper end of the outer conductor 36 to the raceway face 41 of the upper outer conductor 16 is very close to reduce leakage out of the joint. The dished shape or conical surface of the raceway 41 further increases the shielding efiiciency of this arrangement. While there is some capacitive efiect available to help smooth out the discontinuity from the lower to the upper outer conductors, the actual circuit relied on is the ohmic contact of the balls 42 in the'circuit previously related.

The inner conductor circuit runs from the center con ductor of cable 13 -to the upper center conductor 15, through the contact ball to the lower center pin 25 and thence to the center conductor of the lower cable 23.

Spring 34 maintains pressure across ball 30 to ensure reliable contact therewith.

Upon rotation of'mounting flange 10 relative to the other flange 11, the upper raceway 41 moves relative to the lower raceway 39 as does the upper pin 15 move relative to the lower pin 25. The halls turn, twist or roll and maintain contact with little variation of the impedance across the joint. The joint is capable of full continuous rotation up to moderate speeds, displaying low stiction and good voltage standing wave ratio.

As exemplary only, a rough preproduction model of the invention having an internal bore of the outer conductor of .375 inch, an outer diameter of the center pins of .050 inch, a spacing between the upper end of the lower outer conductor and the raceway 41 of approximately .015 inch, and an over-all fiange-to-fiange spacing of approximately 1 inch connected .080 coaxial cable having 52 ohm impedance to exhibit a voltage standing wave ratio ranging from 1.07 at megacycles to 1.15 at 400 megacycles. The long time stability of the joint is excellent. The torque required to turn the joint is very low. Production models enjoying closer physical tolerances show even better characteristics.

Construction of the conductors, the balls, and the hyperboloid spring of hardened beryllium copper gives satisfactory service. For superior conductivity and low motion applications, plating of the raceways 41 and 39 with silver enhances the operation of the system. Where wear is a factor, hard chrome plating of the raceways 39 and 41 is advisable for a long reliable service life.

Although this invention has been described with respect to particular embodiments thereof, it is not to be so limited because changes and modifications may be made therein which are within the full intended scope of the invention as defined by the appended claim.

We claim:

A rotatable coaxial joint comprising a first center conductor, first insulating means mounting said first center conductor, a second center conductor, a conductive ball positioned between said center conductors, a second insulating mounting means, a third insulating mounting means, spring means between said second and third mounting means, said second center conductor being mounted in said third insulating means whereby said spring means urges said second center conductor against said first center conductor and said ball, a first outer conductor coaxial to said center conductors, the inner diameter of said outer conductor having a predetermined radius, a surface of said outer conductor having a raceway, said first outer conductor being mounted on said first insulating means, a second outer conductor, said second outer conductor being coaxial to said center conductors, hyperboloid spring means, means mounting said hyperboloid spring in conductive relation to said second outer conductor, a first portion of said spring means forming a raceway opposing said first raceway, a plurality of conductive balls between said raceways, said first and second insulating means being mounted on separate bodies whereby rotation of one of said bodies relative to the other about the axis of said center conductors is accommodated by said rotatable joint with continuity of its characteristic impedance.

References Cited in the file of this patent UNITED STATES PATENTS 

